“I’m extremely disappointed about the Koch network being off the playing field,” Randy Kendrick, a leading Koch donor from Arizona, told the Guardian about the absence of anti-Trump efforts. “This country does not believe in rule by strongmen or cult personality figures.”

After pushing for a Koch network blitz against Trump, the conservative network’s top political operative, Marc Short, left last month to advise the Rubio campaign and consult for several Senate and gubernatorial candidates, some of whom are likely to get Koch network backing too, say two GOP sources.

“Marc Short felt strongly that something had to be done about Trump, [and] advocated for getting involved,” one conservative said.

While the Koch network’s leadership opted not to attack Trump – due to divisions over alternative candidates, fears an attack would backfire and wariness about getting involved in the election at the primary stage – some of its individual donors have plowed over $4m into the anti-Trump Super Pac Our Principles, including the Ricketts family, which owns the Chicago Cubs; hedge fund mogul Paul Singer; Randy Kendrick, whose husband Ken is an Arizona Diamondbacks co-owner; and billionaire Stan Hubbard.

Super Pacs are political groups that can accept unlimited donations from corporations, individuals and unions to run ads for and against federal candidates, and were spawned by a 2010 supreme court ruling and another court decision that together overturned decades of campaign finance rules.

Kendrick, a top fundraiser for Rubio’s campaign, said that she tried to encourage efforts against Trump by the Koch network, which plans to spend an astonishing $900m this election cycle on political and issue advocacy efforts, the vast majority of which will be spent by not-for-profit groups that don’t have to disclose their donors. But donors in late January learned of a network decision against launching Trump attacks at a Koch policy and fundraising retreat in Indian Wells, California, which drew a few hundred donors who back the network’s many free-market, small-government projects.

“They’re my main peer group and they’re not involved,” Kendrick said about the Koch network. “Trump is a poor vehicle for bringing the greatness of our country back and voters deserve to know the facts about him. There are moral problems with this guy. I’m disappointed with the Kochs, but I’m more disappointed with the national news media, including Fox and Drudge.”

Trump is a poor vehicle for bringing the greatness of our country back and voters deserve to know the facts about him

But some Koch network donors who have donated to Our Principles still support the network’s decision not to attack Trump. Minnesota broadcasting billionaire Stan Hubbard, who gave $10,000 to the anti-Trump Super Pac, said he did so because “Trump is a liar. I don’t like people who tell lies. I like truth tellers.”

But Hubbard added that the Koch network was “smart not to get involved” in attacking Trump because network donors were divided about their preferred candidates. At the January retreat, about half backed Rubio and a quarter supported Cruz with the rest split between other candidates, say network donors.

The initial fundraising for Our Principles was fueled by a $3m donation from Marlene Ricketts, the wife of TD Ameritrade founder Joe Ricketts. The Super Pac ran about $2m in ads in Iowa, where Trump placed second in the state caucus. By 14 March, according to FEC records, the Super Pac’s total spending this year on anti-Trump ads, phone and mail was more than $12m, much of which was targeted at voters in Florida, Ohio, Illinois and Missouri before they hold primaries on Tuesday.

Trump last year derided the Koch network’s efforts to influence politics and their ties to several other major candidates, including Rubio and Cruz, who were invited to attend two Koch donor retreats in 2015 and make presentations to the network’s donors.

Last summer Trump tweeted: “I wish good luck to all the Republican candidates who traveled to California to beg for money, etc from the Koch brothers. Puppets?” Trump did not receive the Koch network’s coveted invitation.

David Koch. A spokesman for the umbrella fundraising hub of the Koch network said: ‘We have never engaged in a presidential primary, and we have no plans to do so now.’ Photograph: Carlo Allegri/Reuters

The Koch network’s decision to refrain from Trump attacks, despite widespread dislike of the real estate mogul’s views on taxes, trade and other issues, stemmed from several factors besides the divisions among donors: there were concerns that any large TV ad effort could backfire or be ineffective, and more broadly that they had not been involved in presidential primaries before.

Mark Holden, the chairman of Freedom Partners Chamber of Commerce, which is the umbrella fundraising hub for the Koch network, said no precedent existed for Trump attacks in the primaries. “First of all, we never have engaged in a presidential primary, and we have no plans to do so now,” Holden said in an email.

But one donor said the network had serious concerns that an effort could “bounce back” and hurt the Koch network given Trump’s penchant for launching tough attacks against critics. When Trump learned last month that Marlene Ricketts had given $3m to Our Principles, he tweeted ominously that her family “better be careful, they have a lot to hide”.

Nonetheless, Short was raising alarms about Trump inside the conservative political network, which had been riding high after helping the GOP recapture the Senate in 2014 and was looking at spending a staggering $889m this election cycle. Roughly $300m of those funds are said to be for political activities including Senate campaigns.

Short served as president of Freedom Partners since 2012, and was widely considered its leading political operative. While Short was pushing for anti-Trump efforts, the Rubio campaign started to woo him, which culminated in his decision to leave Freedom Partners, say two sources.

When the network “decided not to intervene”, Short opted to move on, a conservative source said.

But Short, who was making just under $800,000 a year at Freedom Partners, has a political consulting gameplan beyond Rubio, whose campaign appears to be on its last legs.

Two sources tell the Guardian that Short has a number of Senate and gubernatorial clients he is now advising, including Governor Mike Pence of Indiana, a favorite of Koch donors who Short once worked for when Pence was a US representative.

Holden, whose primary job has long been general counsel to Koch Industries, has hailed Short as a “valued leader” and in an email stressed that he left behind a “strong leadership team that is already strategizing, planning and advancing our mission”. Holden noted too that ads from network groups backing “at least” three Senate candidates will be running soon.

But some conservatives familiar with the Koch network believe Short’s sudden departure leaves a gap in the political operation at a time when spending and political plans for 2016 are being hashed out.

“The Koch political machine has more money than muscle and Marc Short’s separation will leave donors wondering who will run the political strategy,” said one GOP operative familiar with Koch world. “Marc’s departure leaves a significant void. The leadership of Freedom Partners has very limited political experience,” added another conservative source.

Big question marks remain about what the Koch network will do in the general election if Trump wins the nomination.

Holden sounds a cautious note. “We have made clear that we would only get involved in the general presidential election if one or more candidates presented a positive vision for addressing the issues that we believe are best for this country and if they advanced policies that would reverse the trajectory of what has become a two-tiered society.”

Stan Hubbard predicts that in a contest against Hillary Clinton, “I think they’ll back Trump rather than Clinton”. But he added quickly: “Their hope is that Trump doesn’t get nominated.”